Subdomain is an extension of your main domain. For example if your
domain is example.com, you could create subdomains singapore.example.com
and tokyo.example.com. These subdomains could be accessed just like
your main domain.

Our email system is also subdomain aware. You can create email for
subdomains hosted on your account. Based on the example above, you could create
john@singapore.example.com and john@tokyo.example.com, both are different
users.

As you can see, subdomain is a nice way of organizing your your Internet
site in a more professional and structured way.